Some commercial ultrasound scanners use multiple transmit focal zones. One example of such a scanner is the General Electric Logiq 700. However, there is a disadvantage of using the multiple transmit focal zones, namely, changing the focal number affects the frame rate. The greater the number of focal zones, the slower the frame rate.
One proposal for increasing frame rate is described in U.S. Pat. No. 5, 301,674 (Erikson et al., issued Apr. 12, 1994, the "'674 Patent"). FIG. 2E illustrates a method in which the various focal zones are subjected to ultrasound transmit pulses one at a time in increasing distance from the source of the pulses. A guard time is used between each ultrasound pulse, and the timing is such that the echo pulse responsive to each transmitted pulse is received before the next transmit pulse is generated.
The '674 Patent approach does not increase frame rate to the extent needed in modem ultrasound scanners. This invention addresses the problem and provides a solution.